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Reef Carbonates
Definition of a reef
1. Rigid, wave-resistant structure
2. Bathymetric relief above seafloor
3. Form in situ by organic growth (rarely by mineral precipitation)Modern reefs are nearly all constructed by corals in shallow tropical water,
but this hasn’t always been the case
Reef formation is a balance between constructive and destructive processes
Constructive processes:
Growth of organic framework
Binding by encrusters
Sediment trapping by bafflers
Reef limestones composed of:
1. Framework constructing organisms
2. Encrusting algae, microbes, animals
3. Platy “baffling” organisms
4. Cavities filled by sediment and/or cement
Microbial binding much more dominant in Proterozoic-Triassic reefs
Lower Cambrian, Nevada
Baffling by platy algae (voids mostly filled with cement)
Middle Permian, Guadalupe Mountains (New Mexico)
Thin section
Polished slab
Mud mounds: buildups (“bioherms”) with bathymetric relief but no organism framework
Dominated by mud trapped by bafflers or precipitated by microbes, or by cement
Cementation
Common cavity fill, partly responsible for steep wave-resistant reef profile
Lower Cambrian, Nevada
Middle Permian, Guadalupe Mountains (Texas)
Destructive processes:
Wave energy (especially storms and hurricanes)
Bioerosion (dominant destructive process in the Mesozoic and Cenozoic)
Bivalve bioerosion in coral head
Reefs are unbedded structures composed of framestone, bindstone, or bafflestone
Due to bathymetric relief, surrounding beds onlap reef core
Because of rigid framework and cementation, reef foreslope can be extremely steep (to vertical)
Fore-reef grainstones and rudstones
Reef talus